Tensile testing clamp



Sept. 15, 1970 M. s. GADD 3,528,283

TENSILE TESTING CLAMP Fi led Nov. 15, 1967 FIG. 2

lnvenlor A Home y5 United States Patent 3,528,283 TENSILE TESTING CLAMP Michael Stephen Gadd, Finham, Coventry, England, assignor to Courtaulds Limited, London, England, a British company Filed Nov. 13, 1967, Ser. No. 682,101 Int. Cl. G011: 3/00, 3/08 US. Cl. 73-103 3 Claims ABSTRACT OF THE DISCLOSURE This application relates to apparatus for determining the tensile properties of elongated articles, for example filaments, yarns, wires and the like, collectively referred to hereinafter as filaments.

To determine the tensile properties of a filament, each end of the gauge length of the filament is clamped in some way and the clamps are then moved apart until the filament breaks, when the length and load at breakage are measured. With a filament of high tensile strength the clamping force has to be very considerable to prevent the filament slipping in the clamps. However, the use of large clamping forces introduces the risk of damaging the filament by the clamps.

It has been proposed to use as a clamp a cylindrical bollard around which the filament may be wrapped in at least part of a turn so that the tension in the filament is reduced by friction in passing around the bollard to a point where it can safely be clamped between the bollard and a block which can be screwed up to the bollard.

However, the cylindrical bollards used hitherto suffer from the disadvantage that the radius of the bollard has to be relatively large to prevent a large tension differential across the width of the filament, which could lead to premature fracture, particularly around the point where the filament leaves the bollard and the tension is at its greatest. The use of a relatively large diameter bollard decreases the accuracy of the test result because of the slip of the filament over the bollards surface and hence the indeterminate test length of the filament.

According to the present invention a filament clamp for a tensile testing apparatus comprises a bollard around at least part of which a filament can be passed and a clamping member which can be moved towards the bollard for gripping a filament therebetween, at least a part of the surface of the bollard, between the point at which it is to be met by a filament passed over it, when in use in a tensile testing apparatus, and the point on the surface at which the Clamping member can contact the filament for clamping it to the bollard, being a smooth curve having a radius of curvature decreasing in the direction toward the clamping member.

Preferably the radius of curvature of the bollard surface decreases gradually from the point at which it is met by a filament passed over it when in use to the point at which it is met by the clamping member. Preferably, the part of the surface against which a filament is clamped by the clamping member is fiat, this flat clamping part being adjacent to the part of the surface having the smallest radius of curvature.

Patented Sept. 15,, 1970 The ideal shape of the curved part of the surface of the bollard is the polar curve defined by the equation This equation defines a plane spiral, r being the radial distance of a first point on the spiral from the origin, r being a shorter radial distance from the origin to a second point on the spiral, being the coefficient of friction between the filament and the surface, and 0 being the angle subtended at the origin by a line connecting the first and second points. Only a part of a bollards surface can have a shape defined by the Equation 1 but over that part the pressure between filament and the surface is constant.

This can be shown mathematically by consideration of the equation relating the tension in a filament wrapped about an angle 0 of a bollard, with the tension (T where 0:0".

The pressure (P) exerted by the filament on the surface of the bollard, at a point where the radius of curvature is r, is proportional to T/ r this equation being obtained by resolving the tension in the filament into tangential and normal forces. In this embodiment of the invention,

r: r w

therefore from which equation e" can be cancelled showing that the pressure exerted by the filament on the bollard is uniform at any point on their line of contact for a bollard of the shape defined.

A bollard having the ideal shape is difficult to machine accurately and an alternative curved surface which is quite satisfactory for most applications and can readily be machined is the polar curve defined by the equation:

r=r (1-A0) In this case the origin, from which the distances r and r are measured to points on the curve, lies on the curve itself. This is accomplished by choosing the value of the constant A to be 1/1r which make r=0 when 0:180 (i.e., 1r radians).

In this case the pressure between filaments and the bollards surface actually increases slightly over the part of the surface conforming to the Equation 2, but the tension in the filaments does not rise to the point where their premature fracture occurs.

Embodiments of the invention are shown, by Way of example, in the accompanying drawing, both of the figures of which are diagrammatic elevations.

In the figures the end of a test length of a filament 1 is wrapped about part of the surface of a bollard 2 which is rigidly attached to a part of a tensile testing apparatus (not shown). The end of the filament is gripped between the bollard and a clamping member 3 which is urged toward the bollard by a screw 4 passing through a complementarily threaded part (not shown) also attached to the apparatus.

3 shorter than on conventional equipment, leading to greater accuracy of results.

In a bollard where the shape of the curved part of its surface does not conform exactly to one of the polar curves defined by the Equations 1 and 2, but the radius of curvature of the bollards surface decreases in accordance with this invention, it will be appreciated that considerable advantages are still gained in the use of such a bollard because the pressure exerted by the filament on the bollard may be maintained at a higher level than hitherto over their area of contact. It is therefore possible to use a bollard which is smaller than necessary hitherto whilst considerably decreasing the risk of breakage of filaments at the clamps. With the use of a smaller bollard the accuracy of testing is increased bcause the test length of the filaments is more accurately known.

What is claimed is:

1. A filament clamp for a tensile testing apparatus comprising a bollard having a filament receiving surface, said surface having a first portion, less than 360 and more than 90 of arc, curved to the shape of a plane spiral, and an adjacent second portion meeting said curved portion at a tangent to the curved portion at the point where the radius of curvature of said curved portion is minimal, a clamping member and means for urging said clamping member against said adjacnt surface whereby filament to be tested may initially engage said receiving surface at a point on said curved portion, follow a curved path of decreasing radius of curvature in contact with said surface and be secured against said adjacent portion by said clamping member.

2. The clamp claimed in claim 1 wherein the spiral portion of the receiving surface has a shape approximating the polar curve defined by the equation:

where is the radial distance from the origin to a first point on the curve, r is the radial distance from the origin to a second point on the curve such that r is greater than 4, 0 is the angle subtended at the origin by a line connecting the two points, and A is 1/1r.

References Cited UNITED STATES PATENTS 2,372,962 4/ 19-45 Knochel 188-64 FOREIGN PATENTS 610,232 10/ 1948 Great Britain.

RICHARD C. QUEISSER, Primary Examiner J. WHALEN, Assistant Examiner US. Cl. X.R. 242l52 

